A polarizing plate absorbs linear polarization in an absorption axis direction and allows the linear polarization in a transmission axis direction to pass through. Conventionally, a polarizing plate obtained by dying a resin film, such as a polyvinyl alcohol or the like with iodine or a dichromatic dye and stretching the film in one direction has been widely used. However, there has been a problem that the aforementioned polarizing plate is poor in heat resistance and light resistance depending on the kind of the dye or the resin film. Further, there has been a drawback that the polarizing plate has a relatively great thickness.
In contrast, a method for forming a polarizing film by casting a liquid-crystalline coating fluid containing a liquid-crystalline compound on a substrate, such as a glass plate or a resin film and the like to orient the liquid-crystalline compound is known. A lyotropic liquid-crystalline compound is suitable as a liquid crystalline-compound to be used for this purpose.
The lyotropic liquid-crystalline compound forms supramolecular aggregates exhibiting liquid crystallinity in the solution, so that the long axis direction of the supramolecular aggregates is oriented in a casting direction when casting with the application of a shearing stress to the liquid-crystalline coating fluid containing this.
Lyotropic liquid-crystalline compound-based polarizing films do not need to be stretched and are easy to have a greater width. Further, it is possible to reduce the thickness of the lyotropic liquid-crystalline compound-based polarizing films significantly.
Azo compounds having hydroxyl groups in ortho positions of naphthalene rings coupled to azo groups are known as lyotropic liquid-crystalline compounds suitable for making a polarizing film (JP 2007-126628 A).
The polarizing film obtained by casting a liquid-crystalline coating fluid including such an azo compound exhibits absorption dichroism in a wavelength region having wide visible light. However, the polarizing film may have a low dichroic ratio (e.g., the dichroic ratio is less than 10) depending on the molecular structure of the azo compound.